In contrast to pro-apoptotic p53 and its homologue p73, the NF-κB signaling pathway plays an important role in cytoprotection against various pro-apoptotic stimuli such as DNA damage. Under normal conditions, NF-κB exists as heterodimeric complexes composed of p50 and p65 (RelA) subunits and is in the transcriptionally inactive state through interaction with inhibitory proteins such as IκB-α and IκB-β. IκB masks the nuclear localization signal of NF-κB and thereby inhibits NF-κB nuclear translocation.
On certain stimuli, an IκB kinase (IKK) complex, an upstream regulator of the NF-κB signaling pathway, rapidly phosphorylates particular serine residues located in the N-terminal signal response domain of IκB, which is then polyubiquitinated and degraded in a proteasome-dependent manner. As a result, the nuclear localization signal of NF-κB masked by IκB is exposed, leading to the nuclear translocation and subsequent activation of NF-κB. The IKK complex is composed of two catalytic subunits IKK-α (also called IKK-1) and IKK-β (also called IKK-2) and one regulatory subunit IKK-γ (also called NEMO) with a scaffold function.
The relationship between NF-κB and p53 or p73 has been characterized as follows: in response to a primary antigenic stimulation, NF-κB limits the up-regulation of pro-apoptotic p73 in T cells and promotes T cell survival, and however, the precise molecular basis by which the activation of NF-κB inhibits p73 expression is still unknown (Non-Patent Document 1). In response to an anticancer agent doxorubicin, IKK-β, but not IKK-α, activates NF-κB and thereby inhibits the accumulation of p53 at protein levels (Non-Patent Document 2). These results suggest that the activation of NF-κB might suppress apoptosis mediated by p53, p73, or both. In agreement with this suggestion, the presence of bidirectional repression between p53 and NF-κB has been shown (Non-Patent Document 3).
By contrast, it has been reported that NF-κB works as a cofactor of p53 and is required for p53-dependent apoptosis (Non-Patent Document 4). Besides, it has been shown that p53 is a direct transcriptional target of NF-κB, and that the p53-activating signal is partially blocked by the inhibition of NF-κB activation (Non-Patent Documents 5 to 7).
UFD2a (ubiquitin fusion degradation protein-2a), a member of the U-box ubiquitin protein ligase family, was originally identified as an E4 ubiquitination factor. UFD2a catalyzes polyubiquitin chain elongation and allows proteasomal degradation to target the polyubiquitinated substrate protein (Non-Patent Documents 8 and 9). The predicted three-dimensional structure of the U box is similar to that of the RING finger, and UFD2a also acts as E3 ubiquitin protein ligase (Non-Patent Document 9). It has recently been demonstrated that human UFD2a genes are located at the locus 1p36.2-p36.3 of candidate tumor suppressor genes for neuroblastoma and other cancers (Non-Patent Document 10). However, mutation analysis conducted by the present inventors has suggested UFD2a genes are rarely mutated in neuroblastoma and neuroblastoma-derived cell lines. In yeast, Ufd2 is associated with cell survival under stress conditions (Non-Patent Document 8). Through apoptotic stimulation, UFD2a is cleaved by caspase 6 or granzyme B and thereby exhibits remarkably impaired enzyme activity (Non-Patent Document 11).
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[Non-Patent Document 2]: Tergaonkar, V. et al., p53 stabilization is decreased upon NF-κB activation: a role for NF-κB in acquisition of resistance to chemotherapy. Cancer Cell 1: 493-503 (2002).
[Non-Patent Document 3]: Webster, G. A. et al, Transcriptional crosstalk between NF-κB and p53. Mol. Cell. Biol. 19: 3485-3495 (1999).
[Non-Patent Document 4]: Ryan, K. M. et al., Role of NF-κB in p53-mediated programmed cell death. Nature 404: 892-897 (2000).
[Non-Patent Document 5]: Wu, H. et al, NF-κB activation of p53. A potential mechanism for suppressing cell growth in response to stress. J. Biol. Chem. 269: 20067-20074 (1994).
[Non-Patent Document 6]: Sun, X. et al., Identification of a novel p53 promoter element involved in genotoxic stress-inducible p53 gene expression. Mol. Cell. Biol. 15: 4489-4496 (1995).
[Non-Patent Document 7]: Hellin, A. C. et al., Nuclear factor-κB-dependent regulation of p53 gene expression induced by daunomycin genotoxic drug. Oncogene 16: 1187-1195 (1998).
[Non-Patent Document 8]: Koegl, M. et al., Cell 96; 635-644 (1999).
[Non-Patent Document 9]: Hatakeyama, H. et al., J. Biol. Chem. 276: 33111-33120 (2001).
[Non-Patent Document 10]: Ohira, M. et al., Oncogene 19: 4302-4307 (2000).
[Non-Patent Document 11]: Mahoney, J. A. et al., Biochem. J. 351: 587-595 (2002).